A frequency generator of this type has been disclosed, for example, in commonly owned U.S. Pat. No. 3,453,542 to Gunther Hoffmann. Such a generator comprises a number of cascaded denominational (e.g. decadic) switching stages each including a phase-locked oscillator whose control loop includes a phase discriminator receiving a comparison or reference frequency from a source outside that stage. The operating frequency of the oscillator is fed to the associated phase discriminator through a step-down device which may be a selectively adjustable frequency divider or a mixer also receiving a heterodyning frequency. Thus, the oscillator output is stabilized at a frequency which, when modified in the step-down device, matches exactly the comparison frequency supplied to the phase discriminator.
As further described in Hoffmann U.S. Pat. No. 3,453,542, the phase discriminator may be connected in parallel with a frequency-difference detector which goes into action whenever the supplied comparison frequency differs from the stepped-down oscillator frequency by a value greater than that which can be conveniently handled by the associated phase discriminator.
Such a situation occurs, for example, upon the appearance of a denominational carry, i.e. on a changeover from "9" to "0" or vice versa in a decimal system. The large readjustment of the oscillator in this situation results in a brief period of instability which may be acceptable if the resetting involves only one oscillator but which could lead to significant disturbances or even possible malfunctions if it affects two or more oscillators interacting with one another. Thus, a change from 999 to 1000 Hz in a decadic frequency generator, though representing only a small increment, would require large frequency jumps for the oscillators in three of the four cascaded generator stages participating in this operation.
The aforesaid inconveniences are aggravated if the resetting of two or more interacting oscillators occurs at staggered times so that their periods of instability overlap. Such a situation might arise particularly with manual selectors that are independently settable for choosing the digits of different denominational orders.